Modern telecommunication systems require extremely stable and accurate timing devices, with the overall size, operating temperature, power consumption, weight, and ruggedness of the device being critical parameters. Atomic frequency standards have been used in such applications.
Atomic frequency standards use natural resonances within atoms to keep time since the natural atomic resonances are more stable and less sensitive to environmental effects, such as temperature, pressure, humidity, vibration, acceleration, etc., than are macroscopic oscillators like pendulums and quartz crystals. This type of quantum mechanical atomic oscillator, operating at an atomic resonance frequency, is used as a highly stable frequency reference to which the frequency of a variable frequency oscillator, such as a quartz oscillator, can be electronically locked so the high stability and relative insensitivity to environmental perturbations of such a natural atomic resonance are transferred to the quartz oscillator. Such natural frequencies are referred to from time to time herein as "selected reference frequencies," which applies to the resonant frequencies of quantum systems. Quantum systems as used herein, means solids, liquids or gases capable of selectively absorbing or emitting microwave energy, or both, of a selected reference frequency.
The atomic frequency standards usually comprise a voltage-controlled oscillator (VCO), and a physics package and associated electronics that maintains an accurate and stable VCO standard frequency on a long-term basis. The physics package and associated electronics are used to slave the VCO to the selected reference frequency of the quantum system, thereby reducing frequency drift due to oscillator aging and the effects of the environment on the oscillator.
In an atomic frequency standard in which the quantum system comprises a passive (Rb or Cs) gas cell, the physics package includes a light source for optical pumping of the gas, a transparent gas cell situated in a microwave cavity, and a photo detector. The microwave cavity is commonly used to couple injected electromagnetic energy at about the selected reference frequency (i.e., the atomic resonance frequency) to the atomic gas, such as Rb or Cs, within the transparent cell. The microwave cavity is designed to have a microwave resonant frequency substantially equal to the atomic resonant frequency to maximize the effect of the injected electromagnetic field on the atomic gas in the cell. The injected microwave electromagnetic field is generated by frequency multiplication and synthesis from the VCO output.
In operation, the atomic gas within the transparent gas cell is optically pumped by the light from a light source. The microwave electromagnetic energy injected into the microwave cavity interacts with the atoms within the transparent cell and varies the intensity of the light transmitted through the gas cell in a manner dependent on the difference between the injected microwave frequency and the atoms' resonant frequency. The light intensity transmitted through the gas cell is sensed by the photo detector and the variation in light intensity is detected and converted by the photo detector into a physics package electrical signal output. The physics package thus provides a frequency discriminating electrical output signal dependent upon the difference between the injected microwave frequency, which has been synthesized from the VCO output, and the stable atomic resonant frequency.
Examples of other types of atomic frequency standards with microwave cavities are conventional masers [see, for example, M. Tetu, et al., "Experimental Results on a Frequency Standard Based on a Rubidium 87 Maser," Proceedings of the 39.sup.th Annual Symposium on Frequency Control, pages 64-71 (1985)], and a new type of device described in, Aldo Godone, Filipo Levi & Jacques Vanier, "Coherent Microwave Emission Without Population Inversion: A New Atomic Frequency Standard," IEEE Transactions on Instrumentation and Measurement, Vol. 48, pages 504-507 (1999).
The microwave cavity assembly of the physics package is thus the heart of an atomic frequency standard, and substantial efforts have been directed by workers in the field to modify atomic frequency standard physics packages and cavity assemblies to improve their performance or manufacturability without deleteriously affecting their operation.
Previous cavity designs used for such applications include TE.sub.011 and TE.sub.111 right circular cylindrical microwave cavities. These types of cavities, especially the TE.sub.011 cavity, have unacceptably large sizes for use in present-day telecommunications equipment. To reduce the cavity size while maintaining the resonant frequency necessary to excite the atoms in the transparent cell, prior-art designs have used a rectangular cavity. One such design is a TE.sub.101 rectangular cavity partially loaded with a dielectric slab, described in U.S. Pat. No. 4,495,478. Another is a rectangular main cavity with two secondary cavities on opposite sides of the main cavity to produce lumped resonant loading, operating in the rectangular TE.sub.021 mode, disclosed in U.S. Pat. No. 4,349,798. Yet another small size microwave cavity for gas-cell atomic frequency standards is a magnetron-type cavity, described in U.S. Pat. No. 5,387,881. This design uses a cylindrical envelope with four quarter-cylindrical electrodes (coaxial to the envelope) equally spaced around the transparent cell, operating in a pseudo-cylindrical TE.sub.011 mode.
Other microwave cavities are known in the art which use lumped LC resonators and produce longitudinal microwave magnetic fields. Examples include a cavity used for hydrogen masers, as described in U.S. Pat. No. 4,123,727; a helical resonator for use in a rubidium frequency standard, as described in Euro Patent No. 0 330 954, and also in U.S. Pat. Nos. 4,947,137 and 5,192,921. Other resonators of this type are "split ring" (or "loop gap" or "slotted tube") resonators, as described in W. N. Hardy and L. A. Whitehead, Review of Scientific Instruments, Vol. 52, pages 213-216 (1981), W. Froncisz and James S. Hyde, Journal of Magnetic Resonance, Vol. 47, pages 515-521 (1982), and U.S. Pat. Nos. 4,446,429 and 4,633,180. This type of resonator has recently been used in a rubidium frequency standard (paper presented at the joint meeting of the Frequency Control Symposium and the European Frequency & Time Forum in Besancon, France in April 1999 by G. H. Mei and J. T. Liu entitled, "A Miniaturized Microwave Resonator for Rubidium Frequency Standards"). None of these lumped LC resonators operates in a substantially TEM mode.